In thermoelectric circuits it is necessary, for certain measurements, to utilise two identical thermocouples and, whilst keeping one at a reference temperature, to use the other to sense the temperature of an environment to be investigated. Under laboratory conditions, the temperature of the first thermocouple is usually maintained at ice point (zero degrees centigrade) by use of an ice bath, or at some higher fixed temperature by means of a temperature-controlled oven, and the first thermocouple is usually known as the reference or `cold junction`. The other thermocouple is inserted in the environment under investigation and is known as the `sensing junction` or `hot junction`. The known conventional methods for maintaining the first temperature-controlled junction at a constant temperature are satisfactory for use under laboratory conditions, but are impractical in terms of weight, size, cost, power consumption, maintenance, warm-up time and ice replacement or contamination, in many industrial applications, and especially in aircraft and missile applications. The methods of the present invention replace the above methods, as set out herein.
In particular, most practical constructions used hitherto have been of relatively large size and cumbersome in use because of the necessity to include a number of separate items of equipment requiring interconnection with wiring between those items, and also with wiring between the thermocouple junction itself and the indication and/or recording equipment.
In the use of thermoelectric circuits it is desirable to have so-called `linearization`. This term describes the process by which an electrical circuit converts the highly non linear curve of thermoelectric voltage versus temperature to a linear curve of a device output voltage versus temperature.
Each calibration has in practice a unique non-linear calibration curve. By providing linearization, the user does not need to use a table to relate a linear to a non-linear curve, but can simply measure the output of the device and then know that, for example, 1 milli-volt of output voltage is equivalent to, say, 1 degree Centigrade or 1 degree Fahrenheit of measured temperature.
Similarly in RTD (Resistance Temperature Detectors) and thermistors there is need for correction. The output of an RTD is already linear for output versus temperature, but the output curve is unique, and is offset. With this invention, the provision of linearization ensures that the user can measure the output of the RTD and know that, say 1 milli-volt of output voltage, is equivalent to either 1 degree Centigrade or 1 degree Fahrenheit of measured temperature.
Cold junction compensators for use with thermocouple circuits are disclosed in the following documents:
______________________________________ US PATENT DOCUMENTS 1,205,325 11/1916 Clark 136/222 X 1,228,678 6/1917 Johnson 136/222 X 1,411,033 3/1922 Jensen 136/222 X 3,225,597 12/1965 Engelhard 73/361 3,650,154 3/1972 Arnett et al 73/361 3,916,691 11/1975 Hollander et al 73/361 4,133,700 1/1979 Hollander et al FOREIGN PATENT DOCUMENTS 691809 8/1964 Canada 73/361 ______________________________________
Other Publications
Product Bulletin 803-A, Omega Engineering. Inc., 4 pages. Catalog No. C021. Consolidated Omega Devices. Inc., 6 pages.
Avasthy, `Cold Junction compensation for Thermocouple Sensors` Jul. 1973, pp 211 to 212, Institution of Engineers (India), vol. 53, pt 6.
U.S. Pat. No. 4,133,700 of Hollander et al discloses a cold junction compensator which provides the electrical equivalent of an ice-bath reference thermocouple at a selected temperature, for example zero degrees Centigrade. Input connectors, for engagement with conventional thermocouple units, form thermocouple junctions with conductors connected to a battery-operated Wheatstone Bridge circuit adapted to supply an equal and opposite voltage output compensation for variations in the thermocouple junction output at different ambient temperatures.